For the Mac version, John Coates has a download and installation guide at Github. And for the Windows version, Github user cDima has a similar download and guide. All you have to do is download the installer, unzip it and run it. Then you can select "Aerial" as your designated screensaver within your operating system per usual. You can also tweak some settings and get the latest Aerial videos from Apple, make it so a different video plays on each of your multiple displays and set favourites so you always get to see the videos you like. You can find both installers and guides at the links below.